If you have a class in the header of a UI file (i.e. MainWindow.h) and that class has a pointer to the Ui class then slots generated by the designer are put in that class in stead of the Ui class.

Consider this example:

header file:

namespace Ui {
class MainWindow;
}

class MainWindow;
class MyClass
{
public:
  MainWindow *mainWindow;
};

class MainWindow
{
...
};

If you add an action to MainWindow and in the designer select 'goto slot... triggered', the method is added to MyClass and not MainWindow.

result header:

namespace Ui {
class MainWindow;
}

class MainWindow;
class MyClass
{
public:
  MainWindow *mainWindow;
private slots:
  void on_actionTest_triggered(); //this actually should be in MainWindow
};

class MainWindow
{
...
};